Semiconductor memory devices can include blocks of memory cells in a larger integrated circuit or stand-alone integrated circuits. While traditional flash memory devices build the cells in a two dimensional array, some devices may build a three dimensional array of cells. In some three dimensional flash memories, a NAND string may be built vertically, stacking the individual FETs of the string on top of each other, so that the string extends out from the substrate. Such architectures may provide for very high bit densities in a flash memory device. Because the cells are stacked vertically, however, most, if not all, of the cells are isolated from the substrate, involving a different approach to erasing the cells.